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→Problems for uniformitarian theories posed by Umbriel
The orbit of Umbriel is more eccentric than the orbit of [[Ariel]]. Yet Ariel shows signs of past tectonic activity, which some astronomers impute to [[tidal heating]].<ref name=Tittemore>Tittemore, William C. "[http://adsabs.harvard.edu/abs/1990Icar...87..110T Tidal Heating of Ariel]." ''Icarus'' 87:110-139, September 1990. {{doi|10.1016/0019-1035(90)90024-4}} Accessed June 13, 2008.</ref> But Umbriel shows no such signs.
The orbit of Umbriel is more eccentric than the orbit of [[Ariel]]. Yet Ariel shows signs of past tectonic activity, which some astronomers impute to [[tidal heating]].<ref name=Tittemore>Tittemore, William C. "[http://adsabs.harvard.edu/abs/1990Icar...87..110T Tidal Heating of Ariel]." ''Icarus'' 87:110-139, September 1990. {{doi|10.1016/0019-1035(90)90024-4}} Accessed June 13, 2008.</ref> But Umbriel shows no such signs.
In addition, Umbriel poses the same problem for uniformitarian astronomy as do all the other moons of Uranus: its orbit is inclined severely to the ecliptic, though not to Uranus' own equator. How the Uranian system came to have such an inclination has never been explained.
In addition, Umbriel poses the same problem for [[uniformitarianism|uniformitarian]] astronomy as do all the other moons of Uranus: its orbit is inclined severely to the ecliptic, though not to Uranus' own equator. How the Uranian system came to have such an inclination has never been explained.
== Observation and Exploration ==
== Observation and Exploration ==